Pressure leach autoclave circuits are employed in the leaching of ores, concentrates, mattes, alloys, intermediates and the like for the recovery of metals into solution. Once the metals are extracted into solution the value metals can be recovered by hydrometallurgical means, such as purification followed by electrowinning, pyrohydrolysis, crystallisation, hydrogen reduction and other unit operations. In many of these integrated flow sheets (refer to FIG. 1) the pressure leach step is pivotal to the recovery of the value metals from the host material.
The leaching is often accompanied by the release of energy that has to be removed in order to avoid exceeding the design operating temperatures of the autoclave vessel. For example, in the leaching of a synthetic nickel sulphide the reaction could be represented by the following:NiS+2O2===>NiSO4+heat
The heat release from this reaction, as calculated from heats of formation at 25° C., is approximately 790 kilojoules per mole. In a 15 to 17 percent (w/w) concentration of nickel sulphide in water slurry the temperatures could rise to in excess of 200° C. within the autoclave where the normal operating temperature and pressure may be 160° C. and 1200 kPa(g) respectively. In most exothermic autoclave circuits a majority of the energy is liberated in the first compartment with only smaller quantities being generated in the downstream compartments. This heat release is often employed to raise the feed slurry to the design operating temperature. However in many of these circuits the design operating temperature can be exceeded in the first compartment.
The excess heat could be removed by a variety of means, such as quenching by introducing a cooling fluid into the autoclave, internal cooling coils, external coolers, and what is known in the art as “flash and recycle”. Quenching and internal coils have many known practical and process limitations. External coolers avoid many of the problems of internal coils but still may result in scaling of the heat transfer surfaces and significant wear to pumps and isolating valves. In addition, all the above mentioned means are unable adequately to address intermittent surges in mass feed above design and, consequently, temperature excursions above the design value can occur to the detriment of the autoclave's lining systems or the autoclave discharge product.
The flash and recycle system was first introduced by the current inventor in the mid 1980's and employs a first autoclave compartment flash via a flash tank with flash underflow return to the autoclave feed tank. The feed tank serves a dual role; it is both an autoclave feed tank and an external extension of the autoclave's first compartment. In a well designed system, the temperature in the first compartment can be maintained at set point +2° C./−1° C. This temperature control is acceptable for both brick lined and alloy autoclaves. Some of the advantages of this system include optimal utilisation of the installed reactor volume; if required, concentration of the reactor contents through the evaporation of water flashed as steam; and the potenial to use the flashed steam as an energy source elsewhere in the operation. Considered holistically, the flash and recycle process provides the autoclave designer with the maximum degrees of freedom and several process benefits. PCT/AUO2/00584 discloses a flash and recycle system particularly suited to autoclaves operating at low temperature e.g. 120° C. that uses vacuum for the flash.
This invention seeks to enhance the benefits of the flash and recycle system.